Multiple sclerosis (MS) is a disease which significantly affects areas of the central nervous system required for normal neurological functioning. Pathological studies of patients have suggested a viral etiology for this disease, based on similarities to known viral illnesses and the pathological changes observed. Epidemiological studies also support the hypothesis of a viral cause of MS with reported outbreaks of the disease in areas such as the Faeroe Islands. We now believe that our laboratory at the Rockefeller University, as well as other research labs, have sufficient preliminary evidence to warrant a clinical trial in MS using an anti-herpes antibiotic, Valtrex, to determine if the ravages of this disease can be controlled. Specifically, we have demonstrated that the Human herpes type -6 (HHV-6) virus can be detected by immunological methods in post mortem brain specimens of MS patients (oligodendrocyte glial cells), but not in brain specimens from decedents with other diseases. In addition HHV-6 DNA has been detected in MS brain tissue by the PCR technique, but not in controls. Finally, MS patients have a high serum antibody response to the Herpes-6 virus. Valtrex is a drug inhibitory to the HHV-6 virus with a good safety profile and is FDA approved. The general plan of study is to enroll 60 patients into a two-year trial with half randomly assigned to receive the drug and the other half a placebo. The patients will be evaluated over the trial period by two neurologists, one of whom will not know which treatment the patients have been assigned, to assess disease progression and to evaluate disability, as measured in disability rating scales. In addition the frequency of attacks will be monitored and serial brain M.R.I. scans will be obtained to determine the number of brain lesions. Blood and spinal fluid will be periodically drawn to determine the immune response to the HHV-6 virus in the Valtrex study patients and in patients not in the drug trial, but followed at Rockefeller under the protocol "Immunological Studies in MS."